This invention relates generally to xerographic copying methods and apparatus, and more particularly, it relates to the fixing of particulate thermoplastic toner by direct contact with the surface of a fusing member having a novel release surface.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support member, such as a sheet of plain paper, with subsequent affixing of the image thereto.
In order to affix or fuse electroscopic toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrophotographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquify and cause a splitting action in the molten toner resulting in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting", has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the trade name "Teflon" to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of about 1-5 mils and the thickness of the oil being less than 1 micron. Silicone oil, polydimethylsiloxane, which possesses a relatively low surface energy, has been found to be a material that is suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of abhesive material to a rigid core or substrate, such as the solid Teflon outer surface or covering of the aforementioned arrangement. In U.S. Pat. No. 3,934,547 a solid low surface energy coating is applied to a fuser member core by contacting the core with a bar of low surface energy material and a low viscosity release agent is applied to the low surface energy coating to facilitate release of support sheets from the heated core. Exemplary of the apparatus disclosed and claimed in U.S. Pat. No. 3,934,547 is a copper core coated with a fluorocarbon telomer such as Vydax 1000 manufactured by E. I. duPont, and a 60,000 cp silicone oil is applied thereto as a thin layer. The Vydax 1000, a trademark of E. I. duPont, is applied to the copper core from a solid bar of material biased to contact the surface of the copper core. U.S. Pat. No. 3,934,547 is incorporated herein by reference and claims an apparatus comprising a heated fuser structure having a thermally conductive core; means for applying a solid low surface energy material; means for applying a low viscosity release agent to the low surface energy coating; and a deformable backup member forming a nip with the core member. Although this prior art technique reduces the damage from accidental gouging by stripper fingers conventionally employed in such systems and reduces the problems of wear and degradation due to continued operation at elevated temperatures, it is desirable to provide fuser members having abrasion resistant surfaces and good release properties.